The invention relates to cutting elements for rotary drill bits for use in drilling or coring holes in subsurface formations.
In particular, the invention is applicable to cutting elements for use on rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string and an inner passage for supplying drilling fluid to the face of the bit, the bit body carrying a plurality of cutting elements. Each cutting element comprises a preform element, often in the form of a circular disc, including a layer of superhard material having a front cutting face, a rear face and a peripheral surface, a cutting edge being provided by at least part of the junction between the front cutting face and the peripheral surface.
Usually each preform cutting element comprises two layers: a hard facing layer formed of polycrystalline diamond or other superhard material, and a backing layer formed of less hard material, usually cemented tungsten carbide, the two layers being bonded together during formation of the cutting element in a high pressure, high temperature forming press. However, the invention is also applicable to preform cutting elements of the kind comprising a single layer of polycrystalline diamond. Such single-layer preforms may be thermally stable at higher temperatures than two-layer preforms and are therefore commonly referred to as thermally stable preforms.
In either case the preform may be directly mounted on the bit body or may be bonded to a carrier, for example also of cemented tungsten carbide, the carrier being in turn received in a socket in the bit body. The bit body may be machined from metal, usually steel, or may be formed from an infiltrated tungsten carbide matrix by a powder metallurgy process.
Such cutting elements are subjected to extremes of temperature and heavy loads, including impact loads, when the drill is in use down a bore hole. It is found that under drilling conditions spalling of the diamond layer can occur, that is to say the separation and loss of diamond material over the cutting surface of the layer. Such spalling usually spreads from the cutting edge, probably as a result of impact forces. The spalling reduces the cutting efficiency of the element. In severe cases the spalling can lead to delamination, that is to say separation of the diamond layer from the backing layer in the case of a two-layer preform, or separation of the cutting element from the bit surface or carrier to which it is bonded in the case of a single layer thermally stable cutting element.
It has been found that the incidence of spalling can be reduced by bevelling the periphery of the layer of superhard material. Reference in this regard may be made to U.S. Pat. No. Re. 32036. However, the bevelling process is comparatively costly and time consuming, involving as it does the cutting or grinding of a significant amount of the superhard material from the periphery of the layer. Also, bevelling of the edge reduces the sharpness, and hence the cutting efficiency of the cutting edge. The present invention sets out to provide an alternative and advantageous method of treating the layer of superhard material in a cutting element to inhibit spalling.